Princess Lumina and the Queen of Arendelle
by Saeryen
Summary: Three years after Lumina's return to the royal castle, a strange occurrence causes a portal to open between her world and Queen Elsa's. The two discover that Caligo and Hans have teamed up with several new threats to get revenge on Lumina and Elsa, and during their quest to save their kingdoms, the two fall in love.
1. Siren's Curse

Red hair whipped through a stormy, undersea blackness lit by bursts of yellow lightning, the signs of electric jellyfish. The mermaid, however, was uninterested in them. She had more important things to do – namely, finding the one who could help her escape this cursed sea.

The mermaid's shark tail, unique to those of her homeland, violently flailed side to side as she pushed forward. That tall rock was here somewhere among the dark waters, and it was where he was waiting for her.

At last she found what she was looking for. She dove for an opening in the middle of the rock. As she slid inside the cave, gray walls met her black eyes.

"Caligo!" she whispered loudly in a harsh, raspy voice.

A shadowy figure at the room's far end turned to face her. He still wore his old armor, though it had gathered dust in the Sea of Storms. The reason he had kept it on was for his own sake – he had to remind himself of the great wrong he had suffered, and those he would exact his revenge upon.

"Lucasta," Caligo said.

"You called me here," Lucasta replied. "I demand to know why!"

"I told you," the merman said, annoyed.

"Yes. You said you could get me out of this wretched place for good. How will you do that?!"

"Patience, Lucasta," Caligo replied. "I propose a plan to get us both what is rightfully ours."

"I know you want something from me, Caligo. I know of you, and I know you use others to get what's yours."

"Exactly, Lucasta. You have something I do not, yet it is something I have always longed for. A most extraordinary gift."

Lucasta clenched her hands into fists as lightning crackled from them.

"You want me to use my power over storms."

"Indeed." Caligo clasped his hands behind his back as he began to swim upright around the desolate rock that was now his home. "Your gift has always been the pride of the Storm Sirens, and those who possess it have always had all they could ever ask for…friends, glory, reverence…but I could not have those things. I was named after my ancestor, Caliga, who sunk a thousand pitiful human ships with the tempests she made. Somehow, her power was diluted into nothingness along this accursed family line. I was the desperate hope of my parents…alas, I was no luckier than generations before.

"I always envied you, Lucasta. You could command the ocean herself, and I was just a common shark among a mass of children sharing your good fortune. I knew from the beginning I had to find mine elsewhere, and I found it. I married myself to Princess Aerietta, the sister of Queen Lorelei of Seagundia's husband. I had a son, and I knew that son would be my key to something better than any of them had dreamed of. His betrayal was most disappointing, but there was someone else who got in my way. Someone else who will not stand in my way again!"

Lucasta nodded, knowing who Caligo was referring to. When he had returned to the Sea of Storms, surrounded by a swirl of spinning pearls, he had quite publicly shouted his promise to get back at the Seagundian royal family, particularly their long-lost princess whose return had completely shattered Caligo's plans.

"You want to take back the kingdom," the mermaid said. "Fine, I will help you…but you must promise me one thing."

Caligo stared with annoyance, cueing her to continue.

"You must give me mine if I am to get you yours. I have tried to escape these harsh waters for years, and every mermaid kingdom has driven me away. You will ensure that I have permanent residence in Seagundia, where no one may attack me ever, ever, again!"

Caligo sneered.

"It's a deal."


	2. The Storm Strikes

Three long, wonderful years had passed.

Three long, wonderful years since Lumina's dream had come true, that is. After a lifetime of loneliness, the princess of Seagundia had finally made her way out of isolation and home to her parents.

She had watched as the world changed around her, all for the better. When Caligo had been exiled, he'd been sent back to his home waters, never to return "if he knew what was good for him," her father had said. Caligo had been seen in Seagundia since.

With his absence, King Nereus had relieved most of Caligo's soldiers of their duties. The new Trident Squad army was under the leadership of a new general of the guard. She had shown up about a month after Caligo's departure, sought out due to her reputation as a powerful, yet kind and honest mermaid. Her name was Katrena. She had the power to command tempests, one she had inherited from a distant ancestor.

Lumina's family had grown and changed as well. Her cousin Fergis was now engaged to Cora, one of her best friends. Scylla was now the royal potioneer, and Kuda and Spike were known throughout Seagundia as "fish royalty." Lumina's short romance with the prince of the nearby undersea kingdom of Amphitrite was now over; he courted Sandrine these days, but he was still a dear friend of Lumina's, as was his twin sister, Kell.

Lumina had changed, too. She was well known as a benevolent princess who made sure of the well-being of common folk. It was due to her, with the combined help of Scylla and Fergis, that many illnesses now had cures in the form of potions. The distribution of the sulfur lily, which was an antidote to most poisons, was also her idea. With her experience as an explorer, she, accompanied by Spike and Fergis, had scouted out the coral reefs for as many as they could find.

Thus far, it had been a busy week. Sandrine and Scylla were fussing over the decorations for Cora's and Fergis's wedding, trying to gather as many of Fergis's favorite plants that were in Cora's preferred colors of red or orange (some of them had to be dyed), and Katrena was almost panicked with the placement of the guards during the event. Eos, the new captain of the guard, was unable to get anyone to agree on anything, and had pleaded with Katrena for advice. Since then, Katrena had brought it on herself to settle the matter for Eos's sake.

It was thus that Lumina saw the pale, dark haired mermaid sighing heavily as she sat on the edge of an outside balcony.

"Katrena?" Lumina asked.

The startled mermaid jumped, causing her white-gold armor to clank somewhat.

"Oh, I'm sorry," Lumina said. She quickly picked up one of the many large blue pearls she'd been carrying.

"Let me help," said Katrena, gathering half of the pearls. "What are these for?"

"They're supposed to go on the banners at the reception," Lumina replied. "I made them glow – what do you think?"

Katrena smiled. Lumina, understanding, smiled widely back.

"I can take these," Katrena said. She began to reenter the castle. Lumina turned to follow her, but stopped when she saw the surface of the ocean grow dim.

"What's going on?" she said.

Katrena turned. Lumina had let go of her share of pearls. As they floated downwards, she was swimming slowly but purposefully toward the surface.

Katrena dropped her share too. "Princess!" she cried, speeding after Lumina.

Lumina didn't stop swimming, but she did look back.

"Katrena?!" she gasped.

"Princess, you cannot go up there! There is danger, I can feel it!"

"What could be there other than the sun?" Lumina laughed as she pushed forward.

Katrena swam until she was neck and neck with Lumina.

"Princess, I just have a very bad feeling. Whatever is up there, whatever is happening, it is not good and you should not go near!"

Unfortunately for Katrena, her words had the opposite effect than she intended. Her curiosity sparked, Lumina made a bolt for the surface.

 _Splasssh!_

Two mermaids, one fair haired in royal garments, and one dark haired in gleaming armor, broke the surface of the ocean. Right away, they both noticed something was off. The sky was shrouded by swirling, thick gray clouds.

"Work of a storm siren," Katrena said, glaring.

Lumina looked at her quizzically.

"I recognize this power when I see it. I have it!"

Lumina looked into the distance and saw someone. That someone was waving their hands about, undoubtedly commanding the storm.

Just as Lumina started to consider following Katrena's advice and getting back underwater, a bolt of lightning struck the waves not very far from where they were.

Lumina gasped, shocked and horrified. Katrena instinctively grabbed Lumina's wrist and pulled her below the surface.

"Look!" Lumina cried out.

Katrena could see a whirlpool appearing where the lightning strike had hit. It was headed for the castle's highest tower.

"Evacuate!" Katrena called, releasing Lumina. "The castle is under attack by a storm siren!"

Soon, the entire royal family and castle staff were outside, thanks to the quick actions of Eos and Katrena's other guards. Everyone watched, eyes wide, as the whirlpool completely surrounded the highest tower, and they all gasped and cried out when a sapphire colored light came from the tower, covering the whole kingdom for five seconds, before fading away along with the whirlpool.


	3. Season of Calamity

_Meanwhile, in a different realm…_

Queen Elsa of Arendelle was busy. She had a lot in her hands, preparing the castle for the next ball in celebration of the Festival of Spirits and Magic, which fell in the autumn. It was said that this time was when the veil between the worlds – mortal and fairy, living and dead – was thinnest. Grand Pabbie and his family had agreed to put on a spectacular magic show, and people were growing all the squash they could.

"No, please," Elsa was saying to workers at the front gates. "Those green banners are for summer. The red and orange ones are for autumn, to match the leaves."

The servants put down the green tapestries and picked up red ones.

"Thank you," Elsa said sincerely.

Just then, Kristoff ran out of the castle.

"Your Majesty!" he said, panting.

"Please, I've insisted you just call me Elsa!"

"Oh, right. Elsa. Listen, I've been trying to keep Sven out of the vegetable garden. Squash is his new favorite food! He won't stop eating –"

Elsa sighed and lowered her face into her hand. How many times had she said not to bring the reindeer in the castle grounds until all the squash was harvested?

"What's that?" Kristoff suddenly asked.

What was he talking about? Was there another banner mistake or an animal getting in?

But it was neither. Elsa looked up and saw that the sky was growing dark and thick with gray clouds. She was confused; it had been a perfectly clear day. Unless…had she done this? No, no, this was terrible!

Then she heard the thunder and felt the first drops of lukewarm rain.

Elsa breathed a sigh of relief. She wasn't causing this. She couldn't make it thunder or rain.

She turned to the servants.

"Get yourselves inside!" she ordered. "I'll take care of the decorations."

She and Kristoff picked up as many banners each as they could and hurried into the castle, with the rain continuing to pour harder and harder.

"I think we'll just have to put these aside," Kristoff said when they entered the entrance hall. He lay his stack of banners down next to a wall. Elsa followed suit.

She was soaked from head to toe. So was he.

"Elsa!"

Anna entered the hall from one of the other doors. She looked even more soaked than her sister or her beloved. Olaf walked behind her, muttering about how wonderful rain was.

"Anna, are you okay?" Elsa asked.

"Yep, I'm fine," Anna answered. "But it's pouring out there. One minute I was in the garden with Olaf and the ducklings, and the next everyone was quacking without end and scampering back to the pond. Well, the ducks were. Olaf and I don't quack."

Elsa chuckled.

Then the lightning stuck.

 _BOOM_!

The whole hall flashed purple for one moment.

"I think the castle was hit!" Elsa cried out.

She, Anna and Kristoff headed upstairs, running into Kai on the way up.

"I saw it, Your Majesty!" Kai gasped. "A bolt of lightning hit the highest tower!"

"Is it okay?" Elsa asked.

"I checked, Your Majesty. Oddly, nothing appears to be damaged."

Elsa wouldn't have cared if anything was; there was nothing in that tower save for an empty old wardrobe. She was just grateful that an entire section of castle hadn't caught fire.

"Thank you, Kai," Elsa said.

Another bolt of lightning appeared in a window. Elsa noticed that it was not yellow or blue, as one would expect lightning to be, but a deep purple.

 _Strange_ … Elsa thought.

"If you all would excuse me," she said and left. She felt she had to check out the tower.

Elsa made her way through the castle's passages, which she knew well since she and Anna had navigated them quite a bit as children. Finally, she reached the spiral staircase leading up to the highest tower in the castle. Curious, and a bit worried, she ascended.

The tower room was as sparse as it ever had been. It was a round room with four big windows and wooden walls, floor and ceiling. At the far end of it was a dusty old wardrobe, sitting undisturbed by the passage and many huge moments of history.

Except…what was this? From inside the wardrobe Elsa swore she could hear something like the ocean.

The queen of Arendelle stepped forward and opened the wardrobe doors, which for many years had remained untouched.

She gasped.

At the far end of the wardrobe was an elegant palace window, and through it she could see a most remarkable room. Its walls were made of some sort of pink gemstone, and its floor was a dark teal color. In the middle of the room was a pink planter where a magnificent red and orange flower was growing. Strange vines were hanging from the pointed ceiling, and there were three other windows in the room that looked out to a pure blueness.

What was even more remarkable was that where there should have been stairs coming up there was what looked like a lidless trap door.

And then someone rose up through it.

The person was a young woman. She had wavy blonde hair that was only a shade darker than Elsa's and her eyes were as blue as sapphires. Elsa soon noticed that she wore a tiara, so she must be royalty, and in her hair were streaks of pink. Elsa's mouth opened in shock when she saw that the girl was wearing what looked like the top half of a swimsuit bedecked in pearls and jewels, and she let out a shout of surprise when the stranger's bottom half came into view.

She had no legs. Instead, she had the pink, orange and purple tail of a large fish.

Elsa was watching a mermaid in a castle under the sea.

The mermaid turned to the window into which Elsa was staring, and her mouth fell open too.

She swam up to the window and mouthed the words _Who are you_?


	4. Ocean and Sky

Lumina hadn't seen it coming at all.

She'd decided to investigate the tower on her own after Katrena had refused to let her go up there with the guards. But her father, mother and Fergis, for some reason, were all allowed to come (Fergis had declined). Even Aunt Scylla, old as she was, had been given the okay. But Lumina's mother didn't like to leave her out of her sight, and her father, though he tried, was still very protective. King Nereus had convinced his wife that Lumina should be able to go into the city whenever she wanted, but he always wanted her home by nightfall.

What did they know, though? Lumina had survived a swim through the collapsing skeleton of a sea beast, escaped a dungeon on her own, and taken down Caligo singlehandedly. Her parents should've known how capable she was of protecting herself. Only Aunt Scylla seemed to believe she was.

She was prepared to encounter Caligo or his eels in the tower. She was prepared to face sharks or other monsters. She was prepared to face a mean stonefish.

What she never thought she would see was a human woman looking through one of the windows.

Lumina heard a very muddled shout as she entered the tower's topmost room completely, and turned to see where the sound came from.

When she saw who it was, her mouth had fallen open in surprise.

It was a woman – a very beautiful woman. She had blonde hair a shade lighter than Lumina's, and it was in a single braid that fell across her shoulder. Her eyes were big and icy blue. And she wore the most beautiful gown. It looked like it was made of water somehow.

Lumina swam up to the window to get a closer look. The woman wasn't a mermaid – she was human. Lumina had never seen a human before, but she knew they lived above the water and had two legs instead of a tail. The woman had to be human. She wasn't floating, which meant that wherever she was, it was above water. And though her gown came all the way to the floor, Lumina could see shapes beneath it that certainly were not those of a mermaid's tail.

"Who are you?" Lumina asked.

The other woman said something but Lumina couldn't hear a sound.

Lumina knocked on the window. If she broke it, could she get to the other side?

The woman suddenly shouted something. A lot of things. Lumina couldn't make out words, but she could hear the muffled panic in the woman's voice. Why –?

Then she remembered. Humans couldn't breathe water.

If Lumina broke the window, she would probably send the whole ocean into the other side.

* * *

"My name is Elsa, Queen of Arendelle," Elsa had replied to the mermaid's question. In response, the mermaid had started banging on the window.

Was she trying to break it?

"No! Stop!" Elsa shouted. "You can't! The ocean will come in and drown everyone!"

The mermaid withdrew her hand with a guilty, "I-should-have-known" look on her face.

Elsa sighed in relief.

"Who are you?" she asked.

* * *

Lumina saw the woman let out a sigh.

She looked up. The woman was speaking to her again, but Lumina couldn't hear words.

* * *

"Who are you?" Elsa asked again.

* * *

This time, Lumina kept her eyes on the woman's face. She could make out the words _Who are you_?

Lumina didn't want to try to say anything, because she didn't want the woman to misunderstand what she said, given that Lumina thought she probably could only read her too and not hear her very well.

She summoned pearls from the shell she always wore and spelled it out, one word at a time.

* * *

Elsa watched as the mermaid bent the pearls to her will, arranging them into a word.

"Lumina."

Before Elsa could think about anything else, the word "Lumina" rearranged itself into a new word: "princess." Then "princess" turned into "of," which turned into "Seagundia." After "Seagundia," all the pearls flew into a shell the mermaid was wearing near the top of her tail.

Elsa guessed that this meant Lumina hadn't heard her name, if Lumina hadn't said hers out loud. She would need some parchment…

Then she remembered her ice powers.

Elsa made words out of ice.

"I AM ELSA, QUEEN OF ARENDELLE."

Elsa saw Lumina gasp.

* * *

Lumina was astonished. She had never seen such a thing before.

That was frozen water. She knew what it looked like; in the winter a sheet of it covered the ocean's surface, so no one could go above. This woman, Queen Elsa, seemed to have the same powers over water as Lumina did over pearls.

How could she be over there? There was no room beyond the window, just the outside of the castle tower. And somehow she was above water, but the tower was completely submerged.

It had to have something to do with the whirlpool that had hit the castle.

She did know that she couldn't very well communicate with Queen Elsa this way. She wanted to meet her and talk to her properly; it was the only way of figuring out what exactly was going on.

She formed her pearls into the words "I'LL BE BACK," then swam down the tower and towards the ballroom. She had to tell her parents about the phenomenon.


	5. Collision of Realms

"Stay here, please."

Those were the last words Lumina had heard from her mother before she and practically everyone else had gone up the tower to investigate Lumina's findings. Well, not _everyone_ else, just her father, Katrena and Eos. But Lumina was the crown princess; was this not her affair too?

She would not have it. She would not.

So, with her knowledge of the secret passages she had gained almost immediately after arriving at the castle, Lumina snuck her way up to the tower.

When she found her way there, she kept hidden behind the large pots of kelp. She didn't want to get sent away.

"There's something there!" Eos was saying, her brown hand pressed against the window.

Katrena took a look to the left of the room on the other side.

"I think the blackness we saw earlier was merely the inside of this wardrobe. It was shut. The question is, who's opened it?"

"Perhaps we might be able to ask, if only we could get there," the king mused.

Just then, about six people came into view. Lumina recognized Elsa, smiling at just how beautiful she was. With her was another girl who had red hair like Cora's, a tall, buff blond man, and three men who looked like they were royal soldiers.

Everyone at the window tried at once to speak, but Elsa stopped them when she summoned ice, causing them all to stare in awe. With the ice, the human queen formed the words _Where's Lumina?_

"I'm right here!" Lumina said loudly, swimming up to the window.

"Lumina!" King Nereus scolded. "What did we tell you?!"

"Never mind that," Lumina replied somewhat coldly. "We need to find out how to get over there."

"Your Highness," Katrena began, "with these tridents we may be able to break –"

" _Don't_!" shouted Lumina. "If you do that, the ocean will come through and they'll drown!"

Katrena shrugged, her expression apologetic. Of course they would; she should've known that.

Lumina thought. Perhaps it would be better to get the humans over to her side. But how to do that without them drowning in the sea?

Then a thought struck her.

"Aunt Scylla!" Lumina cried, smiling.

"What about her?" Eos asked.

"She can make a potion so that they can breathe underwater!" Lumina continued. "I'm going to find her. And Fergis, he'll know what plants to use."

"Lumina, wait!" Lorelei called, but Lumina had sped off to the lower floors of the castle.

Lorelei sighed.

"She's going to get herself in danger if she keeps being reckless like this," she said, embracing her husband.

"She'll be fine, Lorelei," replied Nereus. But he, of course, was just as worried for his daughter.

"Um, Your Majesties?" Katrena said. "The blonde woman on the other side – she's doing something!"

They turned, and saw that Elsa had poked a sharp piece of ice through the window's border. She moved the ice, cutting half of the window from the border excepting the very top and bottom. She did the same with the other half.

"Doesn't she know –" Katrena began, but her thought was interrupted as she pressed against the window, swinging it open as if it were on a hinge in the middle, which was exactly what Elsa had done.

Katrena screamed as she fell to the other side. Eos, Nereus and Lorelei rushed to her aid, but Elsa slammed the window shut, containing the rest of the ocean.

Katrena gasped. And gasped again. There was no water here, only air! She couldn't swim in air. She had no legs and couldn't walk. She screamed again.

"Get her a chair," Elsa ordered her soldiers.

"Who are you?" Katrena asked.

"My name is Elsa," Elsa replied. "I am the queen of this land, Arendelle. What is your name?"

"Katrena, general of the guard of Seagundia."

The soldiers came back with a chair. Elsa and the red-haired girl helped Katrena sit on it.

"Do you know how this happened?" Elsa asked.

"Not exactly. All I know is that a Storm Siren – a merperson who can summon storms – attacked our kingdom today. Then, an undersea whirlpool hit the tower where that window is. And suddenly, it became a portal to another realm!"

Elsa sighed.

"This tower was hit by lightning. It couldn't have been ordinary lightning; it was purple and the storm came out of nowhere! I think a person with similar type of powers to me had something to do with it."

"Creation of ice?"

"No. I think they're a summoner of storms, like the one who attacked you."

* * *

Deep in the royal kitchen cellar of Seagundia, a young merman was eyeing various kelps and seaweeds through his magnifying glass. He handed some to an old mermaid, who put them into a cauldron right over an underwater heating vent.

"I hope this works," Fergis sighed. "I haven't heard of anyone making something like this before."

"What did you find?" Lumina asked.

"All these," Fergis said, indicating a row of blue and purple kelp, "are used by healers to heal breathing afflictions. And these" – he eyed white flowers – "are used for thinning out water pressure, you know, like if you get a merberry nectar stain on your clothes; it's thicker than water and hard to get out without the juice of these flowers."

"Our chances of being successful are better than they could be," Scylla said as she stirred the potion. "I've looked at every book in the library on this subject – well, similar subjects. As Fergis pointed out, we're the first ones to do this!"

Soon, a light blue steam rose up from the cauldron.

"I think this is as done as it can be," Scylla said. She bottled the potion and handed it to Fergis.

"How will we know if it works?" Fergis wondered out loud.

* * *

"I don't like this, Fergis," said Lumina as they swam up to the nearest shore. "I don't want to just force some land animal to drink this and then stick them in the ocean!"

"Don't worry," Fergis said. "Lumie, if it doesn't work, the animal goes back on land."

"But what if it gets poisoned?"

Fergis opened a bag on his belt.

"Relax, I brought every antidote known to merfolk."

Up on the shore, a cat pawed at a bunch of seaweed. The cat was white, with brown splotches, and its ears were bent forward. It was sniffing at the seaweed like a dog. But the merpeople, who had never before seen either a cat or a dog, thought nothing odd of it.

From underwater, Fergis opened the potion bottle and let the potion surround the seaweed.

Intrigued by the glowing blue seaweed, the cat jumped in the ocean and sniffed the seaweed. He then started to chew on it, once again like a curious dog.

Then, also unusually for a cat, he suddenly wanted to go for a swim.

The cat breathed in and dipped his head underwater. Just then, he suddenly stopped holding his breath.

What was this? The water was breathable, like air. Excited, the cat let out a sharp bark.

Fergis clapped his hands.

"It worked!"

Gently, he picked up the cat and put him back on the shore.

The cat, who was no ordinary cat, but one who favored the speech and lifestyle of a dog, decided he had to inform his best friend of what had happened. Said best friend was a human – a queen visiting a seaside land, far away from her own country of Dulcinea.

* * *

"I can't open it!" Elsa cried. "I'm sorry, Katrena. I can't get you back over there without flooding this room. I'd have to carry you over there, and the window would be open for too long – oh, Lumina's back!"

For she had turned toward the window and seen the mermaid princess reappear, with a young brown-haired merman who looked like a prince alongside her.

Elsa saw Lumina talk with another mer-soldier, a black-haired, brown-skinned mermaid with a pale red tail. After that, Lumina opened the window slightly and slid a bottle through, immediately closing it.

Elsa picked the bottle up.

"Anna, look!" she said.

Anna eyed the bottle and the note attached.

"Kristoff! Katrena!" Anna cried excitedly. "It says if we drink this we'll be able to breathe water! We can get Katrena back home if we use this!"

"Do you think it's safe?" asked Elsa cautiously.

"Princess Lumina and Prince Fergis are upstanding individuals!" Katrena snapped. "They would never ever hand you poison!"

Elsa still looked worried, but she slowly brought the bottle of potion to her lips and drank some.

"Looks like you're still alive, Elsa," Anna giggled.

Elsa handed the bottle to Anna and quickly stepped through the window. She gasped. What they had said was true. She was breathing the water as easily as air!

With her ice, she formed the words _Get Katrena to the window!_

Anna and Kristoff picked the mermaid up and set her down by the window. They had taken the potion, so they followed Elsa through.

"Good!" said Elsa. "Now, we have to open the window one more time and pull Katrena over here!"

Eos pushed against the window, cracking it open, at which point Anna and Fergis each grabbed one of Katrena's arms.

"Pull!" Elsa shouted.

Quickly, they pulled the mermaid through and slammed the window shut.

Anna laughed.

"We'll have to do something about the big puddle on our side."


End file.
